This invention relates to multiple-wavelength laser diode arrays having quantum well (QW) active regions.
For a laser diode with a quantum well (QW) active region, the wavelength (.lambda.) is dependent upon the threshold current density (.sigma.), as shown in FIG. 1. See for example, Chinn et al., IEEE Journal of Quantum Electronics, 24, 2191 (1988) whose contents are herein incorporated by reference. Basically, as the threshold gain increases, the threshold carrier density (and therefore the threshold current) must also increase corresponding to greater filling of the states in the conduction band and the valence band. Therefore, at higher carrier densities, the spectral range over which the gain is positive extends out to higher energies. And due to the gain's spectral broadening (associated with intraband carrier scattering), the gain peak shifts with the QW carrier density to higher energy (shorter wavelength). Furthermore, when the bands fill enough so that the n=2 state begins to deliver gain, the gain at the n=2 transition wavelength can be greater than that at the n=1 (ground state) transition wavelength. This is so because the n=1 state provides gain at the (shorter) n=2 transition wavelength, which adds to the gain provided by the n=2 transition.
This bandfilling up to the n=2 state, can then lead to interesting behavior. As the gain peak blue-shifts continuously with higher carrier densities, it can additionally undergo a large and discontinuous blue-shift. Such a large shift is associated with a shift from the n=1 transition to the n=2 transition.
There is an important need in the art for a multiple wavelength laser diode array providing closely-spaced laser-emitting spots but of different wavelengths that can be optically separated, as in the optical system of certain printers. Multiple-element laser diode arrays are being developed for color and highlight-color xerography. For these applications, the emission from individual elements of the array must be easily and entirely separable, for instance through cross-polarizations or different wavelengths. Different schemes have been proposed to achieve such a structure. One such scheme using band-filling is described in Appl. Phys. Lett, 51 (21), Nov. 23, 1987, 1664-1666. In the scheme described in this publication, a graded-index waveguide separate-confinement heterostructure (GRIN SCH) comprises active regions of a single quantum well (QW) of GaAs divided into twin stripes by waveguide sections formed by Zn impurity-induced disordering. The stripes have different widths, and the generated photons spread into the Zn-disordered regions to different degrees. The high concentration of the Zn acceptors and free holes scatters the photons and thus subjects the lasing stripes of different width to different degrees of losses. As explained in the publication, one stripe can thus be caused to lase at the n=1 transition, and due to band filling, the other (narrower) stripe can be caused to lase at the n=2 transition at a shorter wavelength.
Certain disadvantages are experienced with the scheme described in the publication. One disadvantage is that it is not applicable to other kinds of diode laser geometries, such as a selectively buried ridge (SBR) waveguide laser. Secondly, the loss mechanism described for the effect desired is excessive. As noted in the publication, the threshold current for both lasers was excessive. Thirdly, no explanation is given in the publication how to apply the described scheme to a different material system, specifically the AlGaInP system which lases at wavelengths that are particularly desirable for certain printing applications.